Sundseth, 31, said he believes that he obtained the card from one of four Christian activists who, he said, regularly file Freedom of Information Act requests with federal agencies for correspondence dealing with Christian issues. He said these friends, whom he would not identify, send such letters to him and that he often writes to the senders.

''We are indeed, like it or not, a 'Christian nation' as more than 85 percent of adult Americans consider themselves 'Christians,' '' Sundseth said in the letter to Leib, which Sundseth mailed from his Alexandria, Va., home. ''This country was founded by Christians who were escaping the same kind of small-minded tripe you espouse.

''The framers of the Constitution attempted specifically to anticipate those of your ilk who would try and abridge the very rights of freedom to worship guaranteed us by that document. . . . You are a truly amazing, pathetic creature.''

Sundseth added: ''P.S. When you die, you will be giving account to Jesus Christ, your creator, who happens himself to be a Christian. I hope you are prepared.''

Sundseth, a GS-13 special assistant at the Inter-American Development Bank, said, ''I write a lot of letters like that. I didn't give up my right to express my opinion when I came to government.''